Machining of simulated defects in pipeline steels: A neutron diffraction study of local stress effects

The most commonly used in-line pipeline inspection tools utilize the magnetic flux leakage (MFL) technique, which is sensitive to the stress state of the pipe wall. Calibration of these MFL tools is often carried out using unstressed pipeline sections containing simulated defects, usually produced by mechanical drilling. There is also strong interest in creating simulated defects in stressed pipe walls, thus simulating corrosion in the field. In this study, neutron diffraction is used to map the local stresses surrounding mechanically and electrochemically drilled holes in a steel plate sample. Holes were drilled in unstressed samples, as well as samples held at a constant 80 MPa uniaxial stress. Mechanical drilling into a stressed sample was found to induce considerable residual stresses around the hole. Conversely, electrochemical drilling did not create a residual stress field in the vicinity of the hole.